A communication scheme, which is the next generation of a WCDMA (Wideband Code Division Multiplexing Access) scheme, an HSDPA (High-Speed Downlink Packet Access) scheme, an HSUPA (High-Speed Uplink Packet Access) scheme and the like, that is, an LTE (Long Term Evolution) scheme has been discussed in the 3GPP, which is a group aiming to standardize the WCDMA, and the specification work is under progress.
As a radio access scheme in the LTE scheme, an OFDMA (Orthogonal Frequency Division Multiplexing Access) scheme has been defined for a downlink, and an SC-FDMA (Single-Carrier Frequency Division Multiple Access) scheme has been defined for an uplink.
The OFDMA scheme denotes a multicarrier transmission scheme in which a frequency band is divided into a plurality of narrow frequency bands (subcarriers), and data is loaded on each subcarrier for transmission. According to the OFDMA scheme, subcarriers are densely arranged on the frequency axis while being orthogonal to one another, so that high-rate transmission is achieved, resulting in the improvement of frequency use efficiency.
The SC-FDMA scheme denotes a single carrier transmission scheme in which a frequency band is divided for each user equipment, and transmission is performed using different frequency bands among a plurality of user equipments. According to the SC-FDMA scheme, since it is possible to easily and efficiently reduce interference among the user equipments and suppress variation in transmission power, the SC-FDMA scheme is advantageous in terms of low power consumption of a user equipment, expansion of coverage and the like.
In the LTE scheme, communication is performed by assigning one or more RBs (Resource Blocks) to a user equipment in both a downlink and an uplink.
A base station device decides a user equipment, to which a resource block is to be assigned among a plurality of user equipments, for each subframe (1 ms in the LTE scheme) (this process will be called “scheduling”).
In a downlink, the base station device transmits a shared channel signal to a user equipment, which is selected through the scheduling, by using one or more resource blocks. In an uplink, the user equipment selected through the scheduling transmits a shared channel signal to the base station device by using one or more resource blocks.
In addition, the shared channel signal is a signal on PUSCH (Physical Uplink Shared Channel) in an uplink, and is a signal on PDSCH (Physical Downlink Shared Channel) in a downlink.
Furthermore, as a next communication scheme of the LTE scheme, an LTE-advanced scheme has been discussed in the 3GPP.
In the LTE-advanced scheme, performing “Carrier aggregation” has been agreed as requirements. Here, the “Carrier aggregation” represents that communication is simultaneously performed using a plurality of carriers.
For example, when the “Carrier aggregation” is performed in the uplink, since a user equipment performs transmission using different carriers for each Component Carrier, the user equipment transmits an uplink signal using a plurality of carriers.
Furthermore, when the “Carrier aggregation” is performed in the downlink, since a base station device performs transmission using different carriers for each Component Carrier, the base station device transmits a downlink signal using a plurality of carriers.
However, in communication system employing an FDD (Frequency Division Duplex) scheme in which an uplink frequency is different from a downlink frequency, a user equipment has a device called Duplexer.
The Duplexer is parts sharing an antenna in a radio device employing the FDD scheme, and may also be called an antenna duplexer or a demultiplexer.
In general, in a radio device employing the FDD scheme which simultaneously performs transmission and reception, when one antenna serves as a transmission antenna and a reception antenna, a strong transmitted wave may be introduced in a receiver, resulting in the degradation of reception quality.
The Duplexer is a device for preventing an unnecessary part of the transmitted wave from propagating to a next stage circuit, avoiding the degradation of the reception quality due to the introduction of the unnecessary wave to the receiver, preventing the unnecessary wave from being radiated to the air through an antenna, and further extracting only a signal with a desired frequency, and has a function of electrically separating a transmission path from a reception path.
More specifically, the Duplexer performs the above-mentioned function by allowing filters having two different frequency bands to be shared by one terminal (an antenna terminal).
FIG. 5 is an image diagram of the Duplexer.
However, the above-mentioned conventional mobile communication system has the following problems.
As described above, in the LTE-Advanced scheme, the “Carrier aggregation” is performed. In this case, the above-mentioned problem may occur, that is, a transmission signal may degrade the quality of a reception signal.
For example, FIG. 6 illustrates an example in which transmission in an uplink and reception in a downlink are performed by a user equipment when the “Carrier aggregation” is performed.
As illustrated in FIG. 6, since a Duplexer of a second carrier in the user equipment does not consider a downlink frequency band of a first carrier, when transmission of an uplink signal in the second carrier and reception of a downlink signal in the first carrier are simultaneously performed in the user equipment, the uplink signal in the second carrier may degrade the reception quality of the downlink signal in the first carrier.